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From Wikipedia

Fractional distillation

Fractional distillation is the separation of a mixture into its component parts, or fractions, such as in separating chemical compounds by their boiling point by heating them to a temperature at which several fractions of the compound will evaporate. It is a special type of distillation. Generally the component parts boil at less than 25 Â°C from each other under a pressure of one atmosphere (atm). If the difference in boiling points is greater than 25 Â°C, a simple distillation is used.

Laboratory setup

Fractional distillation in a laboratory makes use of common laboratory glassware and apparatuses, typically including a Bunsen burner, a round-bottomed flask and a condenser, as well as the single-purpose fractionating column.



As an example, consider the distillation of a mixture of water and ethanol. Ethanol boils at 78.4 Â°C while water boils at 100 Â°C. So, by gently heating the mixture, the most volatile component will concentrate to a greater degree in the vapor leaving the liquid. Some mixtures form azeotropes, where the mixture boils at a lower temperature than either component. In this example, a mixture of 96% ethanol and 4% water boils at 78.2 °C, being more volatile than pure ethanol. For this reason, ethanol cannot be completely purified by direct fractional distillation of ethanol-water mixtures.

The apparatus is assembled as in the diagram. (The diagram represents a batch apparatus, as opposed to a continuous apparatus.) The mixture is put into the round bottomed flask along with a few anti-bumping granules (or a Teflon coated magnetic stirrer bar if using magnetic stirring), and the fractionating column is fitted into the top. As the mixture boils, vapor rises up the column. The vapor condenses on the glass platforms, known as trays, inside the column, and runs back down into the liquid below, refluxing distillate. The column is heated from the bottom. The efficiency in terms of the amount of heating and time required to get fractionation can be improved by insulating the outside of the column in an insulator such as wool, aluminium foil or preferably a vacuum jacket. The hottest tray is at the bottom and the coolest is at the top. At steady state conditions, the vapor and liquid on each tray are at equilibrium. Only the most volatile of the vapors stays ingaseous form all the way to the top. The vapor at the top of the column, then passes into the condenser, which cools it down until it liquefies. The separation is more pure with the addition of more trays (to a practical limitation of heat, flow, etc.) The condensate that was initially very close to the azeotrope composition becomes gradually richer in water. The process continues until all the ethanol boils out of the mixture. This point can be recognized by the sharp rise in temperature shown on the thermometer.

Typically the example above now only reflects the theoretical way fractionation works. Normal laboratory fractionation columns will be simple glass tubes (often vacuum jacketed, and sometimes internally silvered) filled with a packing, often small glass helices of 4 to 7 mm diameter. Such a column can be calibrated by the distillation of a known mixture system to quantify the column in terms of number of theoretical plates. To improve fractionation the apparatus is set up to return condensate to the column by the use of some sort of reflux splitter (reflux wire, gago, Magnetic swinging bucket, etc.) - a typical careful fractionation would employ a reflux ratio of around 10:1 (10 parts returned condensate to 1 part condensate take off).

In laboratory distillation, several types of condensers are commonly found. The Liebig condenser is simply a straight tube within a water jacket, and is the simplest (and relatively least expensive) form of condenser. The Graham condenser is a spiral tube within a water jacket, and the Allihn condenser has a series of large and small constrictions on the inside tube, each increasing the surface area upon which the vapor co

Vapor-liquid equilibrium

Vapor-liquid equilibrium (sometimes abbreviated as VLE) is a condition where a liquid and its vapor (gas phase) are in equilibrium with each other, a condition or state where the rate of evaporation (liquid changing to vapor) equals the rate of condensation (vapor changing to liquid) on a molecular level such that there is no net (overall) vapor-liquid interconversion. Although in theory equilibrium takes forever to reach, such an equilibrium is practically reached in a relatively closed location if a liquid and its vapor are allowed to stand in contact with each other long enough with no interference or only gradual interference from the outside.

VLE data introduction

The concentration of a vapor in contact with its liquid, especially at equilibrium, is often in terms of vapor pressure, which could be a partial pressure (part of the total gas pressure) if any other gas(es) are present with the vapor. The equilibrium vapor pressure of a liquid is usually very dependent on temperature. At vapor-liquid equilibrium, a liquid with individual components (compounds) in certain concentrations will have an equilibrium vapor in which the concentrations or partial pressures of the vapor components will have certain set values depending on all of the liquid component concentrations and the temperature. This fact is true in reverse also; if a vapor with components at certain concentrations or partial pressures is in vapor-liquid equilibrium with its liquid, then the component concentrations in the liquid will be set dependent on the vapor concentrations, again also depending on the temperature. The equilibrium concentration of each component in the liquid phase is often different from its concentration (or vapor pressure) in the vapor phase, but there is a correlation. Such VLE concentration data is often known or can be determined experimentally for vapor-liquid mixtures with various components. In certain cases such VLE data can be determined or approximated with the help of certain theories such as Raoult's Law, Dalton's Law, and/or Henry's Law.

Such VLE information is useful in designing columns for distillation, especially fractional distillation, which is a particular specialty of chemical engineers. Distillation is a process used to separate or partially separate components in a mixture by boiling (vaporization) followed by condensation. Distillation takes advantage of differences in concentrations of components in the liquid and vapor phases.

In mixtures containing two or more components where their concentrations are compared in the vapor and liquid phases, concentrations of each component are often expressed as mole fractions. A mole fraction is number of moles of a given component in an amount of mixture in a phase (either vapor or liquid phase) divided by the total number of moles of all components in that amount of mixture in that phase.

Binary mixtures are those having two components. Three-component mixtures could be called ternary mixtures. There can be VLE data for mixtures with even more components, but such data becomes copious and is often hard to show graphically. VLE data is often shown at a certain overall pressure, such as 1 atm or whatever pressure a process of interest is conducted at. When at a certain temperature, the total of partial pressures of all the components becomes equal to the overall pressure of the system such that vapors generated from the liquid displace any air or other gas which maintained the overall pressure, the mixture is said to boil and the corresponding temperature is the boiling point (This assumes excess pressure is relieved by letting out gases to maintain a desired total pressure). A boiling point at an overall pressure of 1 atm is called the normal boiling point.

Thermodynamic description of vapor-liquid equilibrium

The field of thermodynamics describes when vapor-liquid equilibrium is possible, and its properties. Much of the analysis depends on whether the vapor and liquid consist of a single component, or if they are mixtures.

Pure (single-component) systems

If the liquid and vapor are pure, in that they consist of only one molecular component and no impurities, then the equilibrium state between the two phases is described by the following equations:

P^{liq} = P^{vap}\,;
T^{liq} = T^{vap}\,; and
\tilde{G}^{liq} = \tilde{G}^{vap}

where P^{liq}\, and P^{vap}\, are the pressures within the liquid and vapor, T^{liq}\, and T^{vap}\, are the temperatures within the liquid and vapor, and \tilde{G}^{liq} and \tilde{G}^{vap} are the molar Gibbs free energies (units of energy per amount of substance) within the liquid and vapor, respectively. In other words, the temperature, pressure and molar Gibbs free energy are the same between the two phases when they are at equilibrium.

An equivalent, more common way to express the vapor-liquid equilibrium condition in a pure system is by using the concept of fugacity. Under this view, equilibrium is described by the following equation:

f^{\,liq}(T_s,P_s) = f^{\,vap}(T_s,P_s)

where f^{\,liq}(T_s,P_s) and f^{\,vap}(T_s,P_s) are the fugacities of the liquid and vapor, respectively, at the system temperature T_s\, and pressure P_s\,. Using fugacity is often more convenient for calculation, given that the fugacity of the liquid is, to a good approximation, pressure-independent, and it is often convenient to use the quantity \phi=f/P\,, the dimensionless fugacity coefficient, which is 1

From Encyclopedia


liquid one of the three commonly recognized states in which matter occurs, i.e., that state, as distinguished from solid and gas, in which a substance has a definite volume but no definite shape. Properties of Liquids In general, liquids show expansion on heating, contraction on cooling; water, however, does not follow the rule exactly. A liquid changes at its boiling point to a gas and at its freezing point, or melting point , to a solid. The boiling point is especially important because, since liquids change their states at different temperatures, those in a mixture can be separated from one another by raising the temperature of the mixture gradually so that each component in turn undergoes vaporization at its boiling point. This process is known as fractional distillation. Liquids, like gases, exhibit the property of diffusion. When two miscible liquids (i.e., they mix without separation) are poured carefully into a container so that the denser one forms a separate layer on the bottom, each will diffuse slowly into the other until they are thoroughly mixed. Liquids, like gases, differ from solids in that they are fluids, that is, they flow into the shape of a containing vessel. Liquids exert pressure on the sides of a containing vessel and on any body immersed in them, and pressure is transmitted through a liquid undiminished and in all directions. Liquids exert a buoyant force on an immersed body equal to the weight of the liquid displaced by the body (see Archimedes' principle and specific gravity ). Unlike gases, liquids are very nearly incompressible, and for that reason are useful in such devices as the hydraulic press. Liquids are useful as solvents. No one liquid can dissolve all substances; each takes into solution only certain specific substances. Molecular Structure of Liquids The molecules (or atoms or ions) of a liquid, like those of a solid (and unlike those of a gas), are quite close together; however, while molecules in a solid are held in fixed positions by intermolecular forces, molecules in a liquid have too much thermal energy to be bound by these forces and move about freely within the liquid, although they cannot escape the liquid easily. Although the molecules of a liquid have greater cohesion than those of a gas, it is not sufficient to prevent some of those at the free surface of the liquid from bounding off (see evaporation ). On the other hand, the cohesive forces between the molecules at the surface of a mass of liquid and those within cause the free surface to act somewhat like a stretched elastic membrane; it tends to draw inward toward the center of the liquid mass, to draw the liquid into the shape of a sphere, thus exhibiting the phenomenon known as surface tension . A liquid is said to "wet" a solid substance when the attractive force between the molecules of the liquid and those of the solid is great enough to hold the liquid's molecules at the solid surface. For example, water "wets" glass since its molecules cling to glass surfaces, whereas mercury does not since the adhesive force between its molecules and those of glass is not strong enough to hold them together. Capillarity is an example of surface tension and adhesion acting at the same time.

From Yahoo Answers

Question:i asked how to get liquod nitrogen, and i got this. i dont speak science.

Answers:its much cheaper to buy it than try to do it at home, you need really powerful compressors and distillation equiptment,

Question:i need a diagram and a flow chart showing the process of fractional distillation of air ASAP please.. google isn't being much help :) Thanks

Answers:This is really quite simple for you to construct as once liquefied, the gases simply boil off in order of their boiling points. You don't say what level you want to do this at eg as normal or are you trying to separate individual rare gases like Kr as well. The necessary detail can be found by searching on the individual gases. The basic stages for your diagram are : 1] Air is filtered to remove dust 2] Water vapour condenses, and is removed using absorbent filters 3]carbon dioxide freezes at 79 C, and is removed 4] It is then cooled in stages until it reaches 200 C. At this temperature it is a liquid. 5] The liquid nitrogen and oxygen are then separated by fractional distillation. 6] nitrogen boils off first at 196 C 7] oxygen then boils off at 183 C

Question:What are the waste products of this separation and do they pose an environmental threat? Why or why not?

Answers:companies like Air Liquide take air and fractionally distill it. N2 and Ar are main components of Air and are separated. ALso O2 and CO2 the "waste" products are the piddly small precentage of other things that were in the air in the first place. They get put back into the air and get mixed in and dont affect the composition of the air. I am a "tree hugger" chemist and I see NO enviromental threat from air separation


Answers:The biggest problem is that the wastes are very cold, so when vented to atmosphere they cause a visible plume from condensation of water vapor. This results in complaints about pollution from citizens, and visits from EPA/OSHA agents who may not know what they are talking about. I was present at a supplier of liquid nitrogen when they had problems in this regard due to venting of unwanted liquid oxygen. A call to the agent's office finally resolved the difficulty. The oxygen, of course, could be a combustion hazard, so venting needs to be unconfined and not near ignition sources. Other waste gasses won't support respiration, so shouldn't be released in confined areas.

From Youtube

Distillation :Check us out at Distillation is a method of separating mixtures based on differences in their volatilities in a boiling liquid mixture. Distillation is a unit operation, or a physical separation process, and not a chemical reaction. Commercially, distillation has a number of applications. It is used to separate crude oil into more fractions for specific uses such as transport, power generation and heating. Water is distilled to remove impurities, such as salt from seawater. Air is distilled to separate its components notably oxygen, nitrogen, and argon for industrial use. Distillation of fermented solutions has been used since ancient times to produce distilled beverages with a higher alcohol content. The premises where distillation is carried out, especially distillation of alcohol, are known as a distillery. Laboratory display of distillation: 1: A heating device 2: Still pot 3: Still head 4: Thermometer/Boiling point temperature 5: Condenser 6: Cooling water in 7: Cooling water out 8: Distillate/receiving flask 9: Vacuum/gas inlet 10: Still receiver 11: Heat control 12: Stirrer speed control 13: Stirrer/heat plate 14: Heating (Oil/sand) bath 15: Stirring means eg(shown), anti-bumping granules or mechanical stirrer 16: Cooling bath.